patch 8.2.1524: no longer get an error for string concatenation with float

Problem:    No longer get an error for string concatenation with float.
            (Tsuyoshi Cho)
Solution:   Only convert float for Vim9 script. (closes #6787)

src/testdir/test_eval_stuff.vim

@@ -135,6 +135,12 @@ func Test_string_concatenation()
   let a = 'a'
   let a..=b
   call assert_equal('ab', a)
+
+  if has('float')
+    let a = 'A'
+    let b = 1.234
+    call assert_fails('echo a .. b', 'E806:')
+  endif
 endfunc
 
 " Test fix for issue #4507